I just finsihed a scope of work for a single family flip that involves rough plubming, rough electrical, minor duct work, a small section of load bearing wall removal, and a conversion of a haff bath to a full (along with a bunch of cosmetic work).

I assume all the ductwork is already in if there is a boiler.

What has worked for me was taking it down to the studs, paint anything that still smells with Kilz, put in new sub-floors, sheetrock on walls and ceilings, and clean/replace ductwork.  

You WILL need to paint everything including walls, ceilings, (and floors in the grow room , clean the furnace, clean the ductwork, and wash all other non-paintable surfaces with a special cleaner to rid the house of the smell after they leave.Im just finally coming to the end of this process and am putting an ozone machine now because there is still a tiny faint smell when i first open the side door.

I talked to the HVAC guy who put the ventless units in this before we purchased and he seemed like there weren't a lot of options without getting into some crazy expensive systems since there is no ductwork.  

An improvement adds to the value of property, prolongs its useful life, or adapts it to new uses.Some examples of improvements include: a new deck, storm windows/doors, wall-to-wall carpeting, duct work, and insullationAs you noted, even items that would usually be treated as repairs and maintenance are treated as an improvement if incurred while "getting the property ready for use."

White fabric tape on ductwork is another spot. https://www.epa.gov/asbestos/information-owners-and-managers-buildings-contain-asbestoshttps://www.epa.gov/lead

All duct work = no good.

I also recommend spraying the space with Lysol.One thing people don't think about is that there is usually a 1 foot high space between floors for duct work, pipes, electrical etc and/or inside the walls that the rats love to claim as their personal space.  

Does that look like a nice bedroom area for someone, sharing it with the furnace and ductwork?